Hypodermic syringes are used to deliver selected doses of medication to patients. The prior art hypodermic syringe typically includes a syringe barrel having opposed proximal and distal ends; the proximal end being generally defined as the end located away from a patient, and the distal end being generally defined as the end located near the patient during use. A cylindrical chamber wall extends between the proximal and distal ends and defines a fluid containing chamber. The proximal end of the prior art syringe barrel is substantially open and receives a plunger in sliding fluid tight engagement with an interior surface of the chamber wall. The distal end of the prior art syringe barrel includes a passage communicating with the chamber. A needle cannula may be mounted to the distal end of the prior art syringe barrel, such that the lumen of the needle cannula communicates with the passage and the chamber of the syringe barrel. Movement of the plunger in direction toward the proximal end (a proximal direction) draws fluid through the lumen of the needle cannula and into the chamber. Movement of the plunger in a direction toward the distal end (a distal direction) urges fluid from the chamber and through the lumen of the needle cannula.
Medication to be injected with the prior art hypodermic syringe often is stored in a vial having a pierceable elastomeric seal. Medication in the prior art vial is accessed by piercing the elastomeric seal with the needle cannula. A selected dose of the medication may be drawn into the chamber of the syringe barrel by moving the plunger a selected distance in a proximal direction. The needle cannula may be withdrawn from the vial, and the medication may be injected into a patient by moving the plunger in a distal direction after the needle has been caused to pierce the patient's skin or by injection into an intravenous tube or other similar device or structure.
Some medication, such as insulin, is self-administered. The typical diabetes patient will require injections of insulin several times during the course of the day. The required dose of insulin will vary from patient to patient, and for each patient may vary during the course of the day and from day to day. Each diabetes patient will establish a regimen that is appropriate for his or her own medical condition and for his or her lifestyle. The regimen typically includes some combination of a slow or medium acting insulin and a faster acting insulin. Each of these regimens may require the diabetes patient to periodically self-administer insulin in public locations, such as places of employment or restaurants. The required manipulation of the standard prior art hypodermic syringe and vial can be inconvenient and embarrassing in these public environments.
Medication delivery pens have been developed to facilitate the self-administration of medication. One prior art medication delivery pen includes a vial holder into which a vial of insulin or other medication may be received. The vial holder is an elongate generally tubular structure with proximal and distal ends. The distal end of the prior art vial holder includes mounting means for engaging a double-ended needle cannula. The proximal end also includes mounting means for engaging a driver and dose setting apparatus as explained further below. A disposable vial for use with the prior art vial holder includes a distal end having a pierceable elastomeric seal that can be pierced by one end of a double-ended needle cannula. The proximal end of this prior art vial includes a plunger slidably disposed in fluid tight engagement with the cylindrical wall of the vial. This prior art medication delivery pen is used by inserting the vial of medication into the vial holder. A prior art pen body then is connected to the proximal end of the vial holder. The pen body includes a dose setting apparatus for selecting a dose of medication to be delivered by the pen and a driving apparatus for urging the plunger of the vial in a distal direction for a distance corresponding to the selected dose.
The user of the prior art pen mounts a double-ended needle cannula to the distal end of the vial holder such that the proximal point of the needle cannula pierces the elastomeric seal on the vial. The patient then selects a dose and operates the pen to urge the plunger distally to deliver the selected dose. The dose selecting apparatus returns to a zero setting upon injection of the selected dose with this prior art medication delivery pen. The patient then removes and discards the needle cannula, and keeps the prior art medication delivery pen in a convenient location for the next required medication administration. The medication in the vial will become exhausted after several such administrations of medication. The patient then separates the vial holder from the pen body, and removes and discards the empty vial. A new vial can be inserted into the vial holder, and the vial holder and pen body can be reassembled and used as explained above.
The above described medication delivery pen is effective and much more convenient for self-administration of medication than the hypodermic syringes that use separate medication vials. However, prior art medication delivery pens have their shortcomings. For example, prior art medication delivery pens do not prevent the patient from dialing a dose that is greater than the amount of medication remaining in the vial. For example, a patient may thus dial a dose of 0.45 ml, when only 0.30 ml of medication remains in the vial. The patient would only discover that fact after the medication was delivered, and the dose set knob was prevented from complete displacement by the set screw abutting the bottom of the vial. The patient would then have to determine the unadministered amount and use another vial to complete delivery of the desired dose. Such a situation would require two injections be made for administration of a single dose. That obvious inconvenience and additional discomfort from a second injection, in addition to the replacement of the spent vial, render such prior art self-administration devices unsatisfactory.
There is thus a need in the art for a medication delivery pen that overcomes the above-described shortcomings of the prior art.